FIELD OF THE INVENTION
This invention relates to composites of paper and plastic film, to corrugated paperboard incorporating such a composite as a liner adhered to the corrugated medium, and to methods of making the composites and the corrugated paperboard. A corrugated paperboard incorporating this composite has a liner component is especially useful as a sheet on which bakery goods are baked. It has been found that this composite liner will withstand temperatures in an oven up to 350.degree. F. for a period of forty to eighty minutes even though the internal temperature of the bakery product will reach an internal temperature of 200-208.degree. F. This composite liner is especially useful because of the excellent release characteristics from the baked product following baking. Use of the composite eliminates the need for and reduces the requirement for the use of grease in the baking process. The bakery product can be baked upon a composite sheet or in a tray made from the corrugated paperboard incorporating the composite as a liner. The invention is also useful in the packaging field in the form of containers, but it finds advantageous utility in the fabrication of other products, such as point-of-purchase displays, wall panels, posters, and the like, where a combination of the unique structural features of the invention and high quality graphics is desired. The invention is particularly felt to satisfy a long-felt need in the area of folding cartons for packaging, whether as primary packaging or secondary packaging, in composite form or as a liner for corrugated paperboard. The moisture barrier characteristics of the composite liner board permits the construction of cartons that can be used to keep a product cold with ice which is retained within the carton as it melts. Corrugated paperboard having a composite liner board component can also be utilized for keeping a product hot for a period of time because of the resistance of the composite to heat and its ability to retain heat within the carton. Although the composite of paper and plastic film is highly useful and advantageous in itself, it is a particularly important characteristic of the composite that it can withstand the conditions in the "double backer" part of a conventional corrugating machine such that it can be combined with a single faced corrugated board to form a double faced corrugated board without material deterioration of the composite which, to my knowledge, has previously been considered impossible as a practical matter. Important characteristics of the composites alone or as incorporated in corrugated paperboard include scuff and scratch resistance, strength, stretch resistance, tear resistance, separation resistance, resistance to "checking" (cracking in a score line) in the forming of blanks for folding cartons, surface smoothness and gloss, graphics quality and durability, fragrance barrier quality, and moisture barrier quality.